1. Field of the Invention
The present invention relates to a read head having a tunnel junction sensor with a free layer biased by exchange coupling with insulating antiferromagnetic (AFM) layers and, more particularly, to such a read head wherein the free layer is located within a track width of the read head and the insulating AFM layers are located beyond the track width.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic tunneling barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel junction sensor for conducting a tunneling current therethrough. The tunneling current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where a sense current is conducted parallel to or in the major film planes (CIP) of the spin valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the tunneling current is conducted through the sensor without magnetic field signals from the rotating magnetic disk. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance to maximum resistance and R is the resistance of the tunnel junction sensor at minimum resistance.
The first and second shield layers or first and second lead layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second shield layers and/or first and second lead layers serve as leads for conducting the tunneling current (IT) through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor. The tunnel junction sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers are slightly separated by first and second insulation layers from the first and second side surfaces respectively of the tunnel junction sensor for longitudinally biasing and magnetically stabilizing the free layer. This longitudinal biasing also maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition.
It should be understood that the hard bias strength of the hard biasing layers is very sensitive to the thickness of the aforementioned first and second insulating layers between the hard bias layers and the side surfaces of the sensor. The first and second insulation layers are usually thick in order to ensure sufficient insulation between the first and second hard bias layers and the sensor. When the insulation is too thick there is insufficient longitudinal field to suppress multidomain activity of the free layer and when too thin the sensor is stiff in its response to field signals. Therefore, an alternative stabilization scheme is proposed without use of insulation between the sensor and the stabilization materials.